1. Field of the Invention
The present disclosure relates to the sensing and configuration of devices, and more particularly, to the sensing and configuration of devices through the use of a reduced current.
2. Description of the Related Art
A number of protocol specifications, like the HART® (Registered Trademark of the HART Communications Foundation) communication protocol, are designed to support digital communications. These digital communications can be used for the measurement of various processes and parameters of various control devices. These digital communications, within these protocol specifications, typically occur over a traditional range of 4-20 milliAmps (mA). Generally, these digital communications provide host control systems with process and diagnostic information associated with a field device. The digital communications can occur as the host control system monitors and controls an industrial process.
One purpose of such protocol specifications is to establish standards so that “hosts” {or “input/output (I/O) masters”} can communicate with field “devices” (“slaves”) developed by different vendors. One subset of the protocol specifications is classified as having “common functions.” “Common functions” requires that the host or device have to meet all standards within this subset of protocol specifications. This allows the host to require only a single interface layer to support a variety of field devices from many different vendors.
However, other components of the protocol specification are classified as “device-specific,” and are defined by the individual device manufacturer. Since at least some of the digital data that will be passed between a host or I/O Master and the field device is specific to the given field device type, it is important to know what kind of field device is connected to the host control system prior to using the field device within an industrial process in real-time.
In other words, due to the nature of device-specific components of field devices, it is necessary to obtain and verify pertinent information regarding the identification of the field device prior to configuration load and execution. Generally, configuration load and execution can be defined as the initialization of the field device for use in the field, and the actual employment of the field device. Pertinent information can include a unique identifier for a given field device, the vendor name for the field device, the firmware revision installed in the field device, the tag name (that is, the pseudonym) for the field device, description of the field device, and the various range limits that the field device can measure or apply. Without the above information derived from the initial start-up of a field device, it is difficult to integrate device specific data into a real-time process control strategy.
Typically, input devices are current sourcing type, and output devices are a current sinking type. A sensor is an example of an input device and a valve is an example of an output device. Employment of a “base signal” of the sourcing current provides at least two functions. It provides the power to charge up, and initially configure, the field devices, and the base signal also is the carrier over which digital information is conveyed. For a current sinking device, the I/O master should drive the current for providing the “base” signal associated with these protocols. Therefore, for output devices, the host or I/O master provides a minimal specified current in order for the digital communication, with aid of the protocol specifications, to function.
Conventional protocol specifications required users to initially load a control configuration (perhaps with the use of wrong initial control configuration), activate the control strategy, and drive the output (sourcing) current to a base amount required for communication with the field device. This occurred over a traditional range of 4-20 mA. Only then could the actual device identification and configuration data be collected. However, as can be appreciated, this could lead to significant errors in implementing an initial real-time control strategy.
An alternative approach, used in other conventional protocol specifications, was to always provide a minimal current of 4 mA. However, this approach is not acceptable, as it is unsafe to power up a field device that is not initially configured. In any event, a user would have no control of the output devices if a problem would cause the field device to render itself unresponsive to the current.
In conventional technologies, to run field devices, current is applied to the field device after configuration, as after configuration it is controllable. However, during a 4 mA power up to the field device, current is applied even without any configuration, and therefore is no way to control the field device. However, if anything goes wrong after applying the 4 mA current, there is no way to control the field device. Another issue is that in this prior art scenario, the minimal current will be omnipresent, which can create safety problems.
Therefore, there is a need to safely and securely establish communication with a field device and acquire the field device identification data using a current modulated signaling technique, prior to a configuration load.